The objectives of this project were the characterization of immunogenic and pathogenic fractions of Coxiella burnetii based on humoral and cellular mechanisms of immunity. A. Lipopolysaccharide (LPS) vaccine. In A/J mice the phase I LPS was nontoxic at a dose of 100 mug gave complete protection against virulent aerosol challenge without a detectable pre-challenge immune response. Western blot analysis of the antibody activity indicated that peptides which co-purified with the LPS induced anti-protein reactions. B. Biological response modification (BRM). Both phase I whole cells and CMR are effective inducers of tumor necrosis factor and interferon. Immunization of C57BL/10 J mice with phase I whole cells induced time and dose dependent immunosuppression without simultaneous increases in T suppressor cells in the spleen, but with the appearance of a new population of immunoglobulin bearing thymocytes. C. Antigenic shift. Morphologic variants of the C. burnetii developmental cycle undergo stage specific antigenic changes of their cell surface detected by immunogold and monoclonal antibody labels. The endospore was nto recognized by the immune response. E. Significance. The phase I LPS may be an effective vaccine performing both as an adjuvant and peptide carrier. The BRM of the humoral and cellular immune responses by several fractions enhances specific and non-specific immunity. Stage specific antigenic changes of morphologic variant surfaces may be a mechanism of evading the immune response and may lead to the development of chromic infections and reactivation of Q fever.